Multi-line table cell support via backslash continuation

[dereuromark]

Summary

Add support for multi-line table cells using trailing backslash (\) continuation syntax. This would allow cell content to span multiple source lines for improved readability.

Proposed Syntax

Row-Level Continuation (Preferred)

Trailing \ at row end continues the entire row on the next line:

| Name   | Description                              | \
|        | This is additional content for column 2. |

Rationale for preference:

• Cleaner visual alignment in source
• Each line is still a complete row structure
• Easier to parse (row-level state, not cell-level)
• Works naturally with colspan (content goes to same logical cell)

Alternative: Cell-Level Continuation

Trailing \ before closing | continues only that cell:

| Name   | A very long description that needs to \  |
|        | continue on the next line.               |

This is more granular but requires tracking per-cell continuation state.

Content Merging

When lines are merged:

• Strip trailing \ and surrounding whitespace
• Join with a single space (like soft breaks in paragraphs)
• Result: "content \n more" → "content more"

Interaction with Related Features

PR #8: Creole-Style Tables

Multi-line cells should work with:

• Colspan (||): Continuation applies to the logical merged cell
• Rowspan (|^): Continuation lines can include rowspan markers
• Header cells (|=): Headers can span multiple lines

|= Feature    |= Long Description Header                 || \
|             |  that spans two columns and two lines    ||

Issue #18: Table Attributes

Row-level continuation should preserve attributes:

| Cell A | Cell B |{.highlight} \
|        | more B |

The {.highlight} applies to the merged row.

Cell attributes on continuation lines could either:

• Merge with previous attributes
• Override previous attributes
• Be an error (simpler)

Recommendation: Only allow attributes on the first line of a continued row.

Alignment

Continuation lines inherit alignment from the separator row:

| Left   |   Center   |    Right |
|:-------|:----------:|----------|
| Short  | A longer \ |    Right |
|        | centered   |          |
|        | paragraph  |          |

Edge Cases

1. Empty continuation cells: | \ | on continuation = skip column (use previous content only)
2. Code spans across lines: Not supported (code spans must be single-line)
3. Escaped backslash: \\| at line end = literal \, no continuation
4. Trailing whitespace: | content \ | (spaces after \) = continuation
5. Last row + caption: \ does not affect ^ caption parsing

Implementation Approach

1. Feature flag: TableExtensions::MULTILINE_CELLS (opt-in, non-standard)
2. Parser changes in BlockParser::tryParseTable():
   • Detect trailing \ before final |
   • Collect subsequent lines until non-continuation row
   • Merge cell contents before creating TableCell nodes
3. Minimal renderer changes: None needed (cells receive merged content)

Why Row-Level is Preferred

Aspect	Row-Level \	Cell-Level \
Parsing complexity	Lower	Higher
Visual alignment	Better	Harder to maintain
Colspan interaction	Natural	Ambiguous
State tracking	Row-level only	Per-cell state needed
Error recovery	Simpler	Complex

Standard Djot Spec

Per the djot specification:

"Contents of table cells are parsed as inlines."

This means:

• Inline formatting (emphasis, links, code) works
• Block-level content (paragraphs, lists) is NOT supported
• Each cell is inherently single-line

So we dont change the specs, we extend it here.
For cases, where the line length would be too exceeded and such a multiline display would help the formatting/readability.

Related

• PR Add Creole-style |= table header syntax #8: Creole-style table extensions (header cells, colspan, rowspan)
• Issue Add attribute support for table rows and cells #18: Table row/cell attributes
• Issue Djot-Extra: Optional Syntax Extension: Definition-Style Metadata #12: Djot-Extra optional syntax extensions

Open Questions

1. Should continuation lines require the same column count?
2. How to handle mixed continuation + non-continuation rows?
3. Should we support \ in caption lines?